Originally, Internet Protocol (IP) providing access to the Internet was designed for stationary users. Therefore, the basic IP concept does not support user mobility: The IP addresses are assigned to network interfaces depending on their location in the network. In fact, the first part of an IP address (NETID) is common to all interfaces that are linked to the same Internet subnetwork. This scheme prevents a user (a mobile host) from being reachable while moving over different Internet subnetworks, i.e. while changing the physical interface.
In order to enhance mobility on the Internet, Mobile IPs for IP version 4 (MIPv4) and IP version 6 (MIPv6) have been introduced by the Internet Engineering Task Force (IETF) in the standard RFC2002 and in the Internet Draft “Mobility Support in IPv6”, 25 Jun. 1999 (work in progress), respectively. A Mobile IP enables a mobile host to change its point of attachment from one Internet subnetwork to another without changing its IP address. The mobile IP introduces the following new functional or architectural entities. For the sake of brevity, the term ‘IP’ will hereafter refer to Mobile IP.
A ‘Mobile Node’ (MN) refers to a host that changes its point of attachment from one network or subnetwork to another. A mobile node may change its location without changing its IP address; it may continue to communicate with other Internet nodes at any location using its (constant) IP address.
A ‘Correspondent Node’ (CN) refers to a peer node with which a mobile node is communicating. The correspondent node may be either mobile or stationary.
A ‘Home Network’ is the IP network to which a user logically belongs. Physically, it can be, for example, a local area network (LAN) connected via a router to the Internet. A ‘Home Address’ is an address that is assigned to a mobile node for an extended period of time. It may remain unchanged regardless of where the MN is attached to the Internet. Alternatively, it could be assigned from a pool of addresses.
A ‘Home Agent’ (HA) is a routing entity that intercepts any packets destined to the mobile node's home address, while the mobile node is away from the home network. The HA encapsulates packets for delivering them to the mobile node, and maintains current location information for the mobile node.
In Mobile IPv4, a ‘Foreign Agent’ (FA) is a routing entity in a mobile node's visited network, which provides routing services to the mobile node while it is registered in that particular network, thus allowing the mobile node to utilize its home network address. The foreign agent decapsulates the packets that were encapsulated by the mobile node's home agent and delivers them to the mobile node. For datagrams sent by a mobile node, the foreign agent may serve as a default router.
RFC2002 defines a ‘Care-of Address’ (COA) for Mobile IPv4 as the termination point of a tunnel towards a mobile node for datagrams forwarded to the mobile node while it is away from home. The protocol can use two different types of care-of addresses: a “foreign agent care-of address” is an address announced by a foreign agent with which the mobile node is registered, and a “co-located care-of address” is an externally obtained local address which the mobile node has acquired in the network. An MN may have several COAs at the same time. A COA of a MN is registered with its HA. The list of COAs is updated when the mobile node receives advertisements from foreign agents. If an advertisement expires, its entry or entries should be deleted from the list. One foreign agent can provide more than one COA in its advertisements.
In Mobile IPv6, there is no longer a need to deploy special routers as FAs. Mobile nodes make use of the enhanced features of IPv6 to operate in any location away from home without requiring any special support from their local routers. Most packets sent to a mobile node away from home in Mobile IPv6 are routed using an ‘IPv6 Routing’ header rather than IP encapsulation, whereas Mobile IPv4 must use encapsulation for all packets. The use of a Routing header requires less overhead for Mobile IP packet delivery from a CN to an MN. To avoid modifying the packet in flight, however, packets intercepted and routed via a mobile node's home agent in Mobile IPv6 must still use encapsulation for delivery. The COA in Mobile IPv6 is the IP address associated with a mobile node while it is visiting a foreign network.
Both in Mobile IPv4 and in Mobile IPv6, the term ‘Mobility Binding’ is the association of a home address with a care-of address, along with the remaining lifetime of that association. An MN registers its COA with its HA by sending a registration request message. In IPv4, the ‘IPv4 Registration Request’ message may be relayed to the HA by the foreign agent through which the mobile node is registering, or it may be sent directly to the HA if the mobile node is registering a co-located care-of address. The HA returns an ‘IPv4 Registration Reply’ message to the mobile node which has sent an IPv4 Registration Request message. If the mobile node has registered by using a foreign agent care-of address, the IPv4 Registration Reply is delivered to the mobile node via the foreign agent. The IPv4 Reply message informs the mobile node about the status of its IPv4 Request. Optional extension fields containing additional information concerning the connection may be included in the IPv4 Registration Request and Reply messages. In IPv6, a mobile node sends a registration request message directly to the HA, and in response to the request message, the HA returns a registration reply message to the mobile node. The registration request and reply messages are included in a ‘Destination Options’ header, which is used to carry optional information that needs to be examined only by the destination node. The messages are called ‘Binding Update’ and ‘Binding Acknowledge’, respectively.
In order to enable full IPv6 functionality of isolated IPv6 nodes in an IPv4 environment, a ‘6over4’ transmission method has been introduced in IETF standard RFC2529. The principle is that IPv6 packets are encapsulated in IPv4 packets for transmission over the IPv4 network between isolated IPv6 nodes.
A real advantage of IPv6 over IPv4 is that the former contains routing optimization by default, i.e. the home agent is involved only when the first datagram is sent to the mobile node on the connection initiation, hence reducing the overhead required. However, an IPv6 network is not available everywhere and not all user terminals are equipped with IPv6 compatibility. Since the addressing schemes in the two versions are incompatible, the IPv4 and IPv6 nodes cannot communicate with each other in a straight-forward manner.